1. Field of the Invention
The present invention relates to a logarithmic amplifier and, more particularly, to a logarithmic amplifier which is easy in level shift and temperature compensation and adapted for integrated-circuit version.
2. Description of the Related Art
FIG. 1 illustrates a conventional logarithmic amplifier. In the figure, 11 denotes an input signal terminal, 12 denotes a resistor for voltage-to-current conversion, 13 denotes an differential amplifier, 14 denotes a diode and 15 denotes an output signal terminal. The voltage-to-current conversion resistor 12 is connected between the input signal terminal 11 and the inverting input terminal of the differential amplifier 13. The anode and cathode of the diode 14 are connected to the inverting input terminal and the output terminal, respectively, of the differential amplifier 13. The noninverting input terminal of the differential amplifier 13 is connected to ground GND and the output terminal of the differential amplifier 13 is connected to the output signal terminal 15.
FIG. 2 shows another conventional logarithmic amplifier. In this logarithmic amplifier, a circuit composed of a diode 16, an differential amplifier 17, resistors 18 and 19 and a constant current source 20 is connected between the output terminal of the differential amplifier 13 and the output signal terminal 15 of FIG. 1. That is, to the output terminal of the differential amplifier 13 is connected the cathode of the diode 16, the anode of which is connected to the noninverting input terminal of the differential amplifier 17. The inverting input terminal of the differential amplifier 17 is connected to ground potential through the resistor 18 and to its output terminal through the resistor 19. The constant current source 20 is connected between a supply voltage VCC and the noninverting input terminal of the differential amplifier 17.
In the logarithmic amplifier of FIG. 1, the potential at the inverting input terminal of the differential amplifier is brought to ground potential by means of its feedback action, and an input voltage Vin at the input signal terminal 11 is converted a current input by the resistor 12. The resulting current flows in the diode 14 so that a forward voltage VF1 is produced across the diode. The voltage is output from the output signal terminal as a logarithmically compressed output voltage Vol. The output voltage Vol is obtained with respect to ground potential as with the input voltage Vin and given by ##EQU1## where q=electronic charge
k=Boltzmann constant PA1 T=absolute temperature PA1 Is1=saturation current of the diode 14 PA1 R1=resistance of the resistor 12
As can be seen from equation (1), the conventional logarithmic amplifier of FIG. 1 suffers from poor temperature-characteristic problems because the output voltage Vol varies with temperature due to a coefficient kT/q and Is1 has great temperature dependence.
In the logarithmic amplifier of FIG. 2, a voltage which is higher than the output voltage Vol of the differential amplifier 13 by the forward voltage VF2 across the diode 16 is amplified by the differential amplifier 17 to produce an output voltage Vo2 at its output terminal. In this case, the current flowing through the diode 16 is a constant current Io from the constant current source 20. Thus, the output voltage Vo2 of the differential amplifier 17 is given by ##EQU2## where R1, R2 and R3 are values of the resistors 12, 18 and 19, respectively, and Is1=Is2.
Assuming here that and the resistors 18 and 19 have different temperature coefficients, the temperature dependence due to the coefficient kT/q is canceled out.
In this case as well, however, the output voltage Vo2 is obtained with respect to ground potential as with the input voltage Vin. For this reason, in order to shift the level of the voltage Vo2 and change the reference potential of the output voltage Vo2, a temperature-compensated complex level shift circuit will be needed. In addition, since the input impedance of the logarithmic amplifier is determined by the resistance of the voltage-to-current conversion resistor 12, a free choice of an input impedance and a high-impedance version thereof are impossible.